This invention relates in general to electrophotography and, in particular, to electrophotoconductive imaging members having multiple layers.
In electrophotography, an electrophotographic plate, drum, belt or the like (imaging member) containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The imaging member is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly to a support such as paper. This imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. A layered photoreceptor having separate photogenerating and charge transport layers is disclosed in U.S. Pat. No. 4,265,990. The photogenerating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements, including narrow operating limits, on photoreceptors.
The numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an anti-curl backing layer and an overcoating layer.
One problem associated with multilayer electrophotographic imaging members is delamination. Since the various layers of a multilayer electrophotographic imaging member contain differing materials, the adhesion of these layers to one another will vary. In particular, it is desirable to provide an adhesive layer between the charge blocking layer and the charge generating layer since adequate adhesion may not be obtained when certain materials are used for these layers.
A number of materials have been provided for the adhesive layer. For example, copolyesters such as du Pont 49,000 resin available from E. I. du Pont de Nemours & Company and Vitel PE-100, Vitel PE-200, Vitel PE-200D and Vitel PE-222 resins, all available from Goodyear Rubber and Tire Company, are commonly employed. With such polyesters, adhesion may be increased in proportion with the thickness of the adhesive layer.
U.S. Pat. No. 4,786,570 to Yu discloses an exemplary electrophotographic imaging member. The electrophotographic imaging member comprises a flexible substrate, a hole blocking layer comprising an amino silane reaction product, and an adhesive layer having a thickness between about 200 angstroms and about 900 angstroms consisting essentially of at least one copolyester resin having the following formula: ##STR1## wherein the diacid is selected from the group consisting of terephthalic acid, isophthalic acid and mixtures thereof, the diol comprises ethylene glycol, the mole ratio of diacid to diol is 1 to 1, n is a number between about 175 and about 350 and the Tg of the copolyester resin is between about 50.degree. C. to about 80.degree. C. The imaging member also includes a charge generating layer comprising a film forming polymeric component, and a diamine hole transport layer, the hole transport layer being substantially non-absorbing in the spectral region at which the charge generating layer generates and injects photogenerated holes but being capable of supporting the injection of photogenerated holes from the charge generation layer and transporting the holes through the charge transport layer.
In general, adhesive layers provide adequate adhesive bond strength linking of the charge generating layer to the charge blocking layer. However, certain charge generating layers do not exhibit adequate adhesion with commonly used adhesive layers. This adhesion problem may be due to the particular constituents of the charge generating layer or to the processes used to produce the layer. For example, charge generating layers containing dispersions of phthalocyanines or benzimidazole perylenes in polymer binders exhibit poor adhesion with adhesive layers. Benzimidazole perylene is a photogenerating pigment of interest because of improved photogenerating characteristics. Further, particles of benzimidazole perylene may be dispersed in a dissolved polymer in solvent system and applied as a dispersion solution coating, a process that avoids cracking of the charge generating layer which may occur upon application of the charge transport layer. However, adhesion as provided by an adhesive layer between a charge blocking layer and a charge generating layer containing benzimidazole perylene, especially in desirable high concentrations, is substantially reduced and the resulting electrophotographic imaging members are highly susceptible to layer delamination during imaging belt machine functions.